1. Field of the Invention
The present invention relates to a developing device, a process unit including the developing device, and an image forming apparatus.
2. Description of the Related Art
FIG. 9 is a schematic diagram of a developing device 30 included in a conventional image forming apparatus, such as a copier, a printer, a facsimile, or a multifunction product (MFP). As shown in FIG. 9, the developing device 30 includes a developing roller 5, a supplying roller 31, a blade 32, a toner hopper 6, and a toner stirring member 33.
The supplying roller 31 includes a sponge layer to obtain toner on its outer circumference. The supplying roller 31 is rotated in the same direction as that of the developing roller 5, so that the supplying roller 31 supplies toner from the sponge layer to the developing roller 5. The blade 32 is, for example, a blade spring made of a metal. An end of the blade 32 is in pressure contact with the surface of the developing roller 5 whereby the toner on the surface of the developing roller 5 is formed into a thin layer having a uniform thickness. The toner stirring member 33 is rotatably mounted in the toner hopper 6. The toner stirring member 33 is rotated to stir toner T contained in the toner hopper 6. The developing roller 5 includes a rubber layer on its outer circumference. The surface of the developing roller 5 is in contact with the surface of an image carrier 2 (photosensitive element). The developing roller 5 transfers the toner from its surface to the surface of the image carrier 2 whereby a toner image is formed on the surface of the image carrier 2.
To prevent uneven image density and image loss, it is necessary to transfer the toner from the surface of the developing roller 5 to the surface of the image carrier 2 at a uniform density. Therefore, the image carrier 2 and the developing roller 5 need to be in contact with each other at a uniform contact pressure. If the developing roller 5 is moved away from the image carrier 2 and is not in contact with the image carrier 2, the toner cannot be transferred from the surface of the developing roller 5 to the surface of the image carrier 2, which results in the image loss. On the other hand, if the developing roller 5 is moved too close to the image carrier 2 and is pressed against the image carrier 2 at a high pressure, this causes a high image density.
Japanese Patent Application Laid-open No. 2006-171295 discloses a technology for biasing a developing roller toward an image carrier by using a biasing member such as a spring, so that the developing roller is in contact with the image carrier at a uniform contact pressure.
FIGS. 10A and 10B are schematic diagrams for explaining movement of the developing roller 5 and the image carrier 2. In FIG. 10A, the rotation axis of the image carrier 2 is located close to the rotation axis of the developing roller 5, and in FIG. 10B, the rotation axis of the image carrier 2 is located apart from the rotation axis of the developing roller 5. As shown in FIG. 10A, the developing roller 5 can be moved toward and away from the image carrier 2, and a bearing 34 is attached to each end of the rotation axis of the developing roller 5. A biasing member 35 such as a spring presses the bearing 34 toward the image carrier 2, so that the developing roller 5 is pressed against the image carrier 2.
Although each of the developing roller 5 and the image carrier 2 is eccentric, a distance between the image carrier 2 (the rotation axis of the image carrier 2) and the developing roller 5 (the rotation axis of the developing roller 5) can be adjusted by moving the developing roller 5 toward and away from the image carrier 2. Specifically, the distance between the image carrier 2 and the developing roller 5 can be made short like a distance D1 shown in FIG. 10A, or can be made long like a distance D2 shown in FIG. 10B. Thus, even if the developing roller 5 or the image carrier 2 does not have a perfect circle shape or is eccentric, it is possible to maintain a uniform contact pressure between the developing roller 5 and the image carrier 2.
Furthermore, if the contact pressure between the developing roller 5 and the image carrier 2 is made small, it is possible to reduce rotary torque of the image carrier 2 and the developing roller 5 and prevent toner deterioration caused by friction between the developing roller 5 and the image carrier 2.
A conventional image forming apparatus includes a process unit having an image carrier, a charging unit, a developing device, and a cleaning unit that are integrally contained in a casing. The process unit is detachably attached to a main body of the image forming apparatus. The process unit is detached from the main body of the image forming apparatus, so that a maintenance work can be easily performed.
FIG. 11 is an exploded perspective view of a process unit 1. The process unit 1 includes the developing device 30, the image carrier 2, and a pair of supporting members 50a and 50b. The supporting members 50a and 50b support the developing device 30 and the image carrier 2. Holes 51a and 51b are formed on lower portions of the supporting members 50a and 50b, respectively, to insert both ends 2a and 2b of the rotation axis of the image carrier 2. Long holes 52a and 52b are formed on upper portions of the supporting members 50a and 50b, respectively. The long holes 52a and 52b extend in a first direction that extends toward and away from the image carrier 2 (the holes 51a and 51b).
Guides 54a and 54b each having a U-shape are arranged on the supporting members 50a and 50b, respectively. The guides 54a and 54b extend in the first direction. Bearings 34a and 34b are arranged in grooves of the guides 54a and 54b, respectively. Each of the bearings 34a and 34b is movable in the corresponding groove. Biasing members 35a and 35b are arranged in the grooves of the guides 54a and 54b, respectively. The biasing members 35a and 35b bias the bearings 34a and 34b toward the holes 51a and 51b. 
The developing roller 5 is arranged on a lower portion of a main body 37 of the developing device 30. Moreover, a pair of projected portions (bosses) 36a and 36b is arranged on upper side surfaces of the main body 37.
To assemble the above components of the process unit 1, the ends 2a and 2b are inserted through the holes 51a and 51b, respectively. Furthermore, both ends 5a and 5b of the developing roller 5 are inserted into the bearings 34a and 34b, and the projected portions 36a and 36b are inserted through the long holes 52a and 52b, so that the main body 37 is attached to the supporting members 50a and 50b. Retaining members 53a and 53b such as screws are attached to ends of the projected portions 36a and 36b that are laterally protruded through the long holes 52a and 52b. Thus, the supporting members 50a and 50b are prevented from being detached from the main body 37.
When the components of the process unit 1 are assembled in the above manner, the developing roller 5 is pressed against the surface of the image carrier 2 by biasing forces of the biasing members 35a and 35b. 
Because the projected portions 36a and 36b and the rotation axis of the developing roller 5 are arranged near opposite ends (the upper end and the lower end) of the main body 37, the main body 37 is supported by the supporting members 50a and 50b in a stable manner.
FIG. 12 is a schematic diagram for explaining a process of attaching the process unit 1 to a main body 60 of the image forming apparatus. The main body 60 includes side walls 61a and 61b facing each other with a predetermined distance, and grooves 62a and 62b are formed on the side walls 61a and 61b, respectively, in the longitudinal direction.
To attach the process unit 1 to the main body 60, the ends 2a and 2b protruding through the supporting members 50a and 50b are inserted into the grooves 62a and 62b. When the ends 2a and 2b are brought into contact with lower ends of the grooves 62a and 62b, the position of the process unit 1 is set with respect to the main body 60.
If the image carrier 2 and the developing roller 5 are rotated when the process unit 1 is attached to the main body 60, the bearings 34a and 34b are moved inside the guides 54a and 54b, so that the developing roller 5 (the rotation axis of the developing roller 5) can be moved toward and away from the image carrier 2 (the rotation axis of the image carrier 2). Moreover, the projected portions 36a and 36b are moved inside the long holes 52a and 52b with the movement of the developing roller 5 toward and away from the image carrier 2.
When the process unit 1 is attached to the main body 60, it is possible that the supporting members 50a and 50b are twisted in directions indicated by arrows A and B in FIG. 11 due to a dimension tolerance or an assembly error of the components of the process unit 1, a dimension tolerance of the grooves 62a and 62b formed in the main body 60, or the like. Specifically, the supporting members 50a and 50b are circumferentially twisted around the rotation axis of the image carrier 2 in opposite directions to each other.
When the supporting members 50a and 50b are twisted with respect to each other, the ends 5a and 5b and the projected portions 36a and 36b are moved with the movement of the supporting members 50a and 50b in the opposite directions. FIG. 13 is a perspective view for explaining a state of the developing roller 5 that is tilted with respect to the image carrier 2. Specifically, the axis line of the developing roller 5 is tilted with respect to the axis line of the image carrier 2.
FIG. 14 is a side view of the developing roller 5 and the image carrier 2 from the axial direction of the image carrier 2 for explaining states of the developing roller 5 that is tilted with respect to the image carrier 2. When the axis line of the developing roller 5 is tilted with respect to the axis line of the image carrier 2, distances D3 and D4 between the rotation axis of the image carrier 2 and the rotation axis of the developing roller 5 at both areas SL and SR are longer than a distance D5 between the rotation axis of the image carrier 2 and the rotation axis of the developing roller 5 at a middle area 5M. Therefore, the contact pressure between the developing roller 5 and the image carrier 2 at each of the areas SL and SR is smaller than that at the middle area 5M. Furthermore, when an amount of twist of the supporting members 50a and 50b is large, or a spring having a relatively small biasing force is used as the biasing member of the developing roller 5, the developing roller 5 cannot be in contact with the surface of the image carrier 2 at the areas SL and SR. As described above, the developing roller 5 cannot be in contact with the image carrier 2 at a uniform contact pressure in the axial direction, which results in the uneven image density and the image loss.
To prevent the uneven image density and the image loss, in a conventional image forming apparatus, a soft rubber is applied to a rubber layer of a developing roller. With this configuration, when the developing roller is tilted with respect to an image carrier, because a middle portion of the rubber layer that is pressed against the image carrier is largely deformed, both ends of the developing roller can be in contact with the image carrier. However, in this technology, there is a limitation on a type of rubber that can be applied to the rubber layer, and because the middle portion of the rubber layer is largely deformed, the high image density is caused at the middle portion of the developing roller.
As an alternative way of preventing the uneven image density and the image loss, the biasing force applied to a developing roller is made large, and a middle portion of a rubber layer that is pressed against an image carrier is largely deformed. However, in the same manner as the above technology, this technology also causes the high image density at the middle portion of the developing roller. Moreover, when the biasing force of the developing roller is large, it is difficult to reduce the rotary torque of the developing roller and prevent toner deterioration.